From VoIP.ms Wiki
Most analogue telephone adapters are intended for push button tone dial use, as the tones can be generated natively by all new telephones.
Most subscribers use recent-model handsets to get features such as call display.
There are, however, still a few old 'phones in use which generate dial pulses. Many are intended to look as if they belonged to another era for decorative purposes or may even be legitimate antiques - such as three-slot coin 'phones from the 1950s or candlestick 'phones from the 1920s.
A few 1980s phones look like they offer the standard tone keypad, but keypresses are turned into pulses internally. There are also a few old alarm panels which dial using pulses only.
There are a few options to keep this equipment working when switching to VoIP.
Analogue telephone adapters
A very small number of analogue telephone adapters support pulse dial:
- Grandstream HandyTone 502 - HT502 has been reported by third-party sites to recognise the pulses, although this feature is not officially documented and appears to have been removed from the later HT702 series. 
- Grandstream GXW4216/GXW4224/GXW4232/GXW4248 also support pulse as of firmware version 184.108.40.206;  these are 16-line (or greater) analogue telephone adapters and aimed primarily at the enterprise PBX market
- Some claim the Motorola VT-1005 to recognise pulses, if you can find one which hasn't been locked to one arbitrary provider.
- MPlink Taiwan claims its SP200 adapter supports both pulse and DTMF tones.
In general, there is no support for pulse dial on standard Cisco (Linksys/Sipura) boxes, which are the most common adapters. There may be a few small manufacturers who support both pulse and tone in some of their devices, but options are limited.
These were small, battery-powered devices which could be held to the handset's microphone; they had a power switch and the standard 12-button tone keypad. These were used mostly to access voicemail (remote playback on answering machines) or automated attendant systems.
In the 1980s and 1990s these were widely carried by local electronics dealers such as the Radio Shack chain, but they're becoming harder to find as most new 'phones can be switched to generate either pulses or tones natively. Some do still turn up second-hand as used equipment.
Pulse to tone converters
These are currently manufactured for a niche market of old 'phone collectors who want to keep the telephone instrument itself historically accurate. Rotary dial appeared in the Bell System in 1919 and tones only became an optional extra (at an added monthly charge) in the mid-1960s, so nostalgia and tones don't mix. Devices like those on www.dialgizmo.com or www.oldphoneworks.com/pulse-to-tone-converters/ are circuit boards with a small microprocessor wired to the analogue line to listen for the pulses and send the corresponding tones.
Private branch exchanges for office telephones vary widely in their support for rotary dial telephones. Many simply don't work at all with any standard 'phone, pulse or tone, as they're designed for whatever handsets came with the system. Others may be tone-only or (if they were manufactured long ago) pulse only. Some models of Panasonic hybrid KSU private branch exchange may be able to convert between pulse and tone in both directions; these were office telephone systems designed to accept either the proprietary stations which came with the system or standard telephones as individual extensions. An analogue telephone adapter would still be needed with this system.
Some Asterisk PBX installations run on desktop-style computers to which cards have been added to connect to individual telephones. The Zapata TDM400 card is one option to add FXS ports on which pulse dial may be enabled (set pulsedial=yes in zapata.conf) although some of the other settings (such as debounce) may need to be adjusted to get this to work. There are even third-party add-ons which add voice recognition to Asterisk; in theory, it may be possible to have the computer ask "Number, please?" much like the manual operators back in 1900.
If all else fails (perhaps because a telephone is a pre-1920s model with no dial at all, from a manual system where picking up the earpiece caused the local operator to ask "Number, please?") one last resort is to configure the analogue telephone adapter so that picking up the handset automatically reaches one pre-set number. No dial, tones or pulses required. A dial plan like <:123-456-7890>S0| (on Linksys ATAs) will dial out to the one predefined number as soon as the telephone is taken off-hook.
An alarm panel which only dials one fixed number (the monitoring station) when the alarm is tripped may work adequately with a "warmline" configuration (the adapter merely waits a few seconds, then blindly assumes the monitoring station is the desired destination number).
When to give up?
Not all equipment can be readily adapted.
A telephone wired for rural party line service is usually just a standard rotary dial instrument, but has the bell wired from one side of the line to ground (the red and green wires are the outside line, the yellow wire is ground). These can be made to work, but won't ring until the wiring is put back to normal.
Sometimes, old 'phones take more ringer power than their modern equivalents. Some online vendors (such as sandman.com) offer devices to boost power on an analogue line to accommodate this, although it's usually an issue only when ringing multiple telephones from the same adapter.
On the other hand, a magneto telephone (the one with a large battery in the bottom and a crank on one side) is not designed for standard lines. These were designed to be wired directly to other magneto telephones. Turn the crank on one and the others ring. Apparently no one had heard of voip.ms in 1876.
A few other non-standard items include sound-powered telephones (which are designed to be wired directly to another sound-powered telephone) and office PBX telephones designed for one specific switchboard. Most of these simply won't work on any standard line.