From VoIP.ms Wiki
Account Settings In order to make and receive calls with this app, you must complete this screen with data provided by your VoIP service provider.
Basic Account Settings
Account Enabled - ON to set account as active. Name - used to identify the account. User name - user portion of SIP Address Of Record (AOR). User@domain is not a valid entry. Password Domain - e.g. example.com or 126.96.36.199 . Description - for yourself
In most cases there is no need to modify these settings.
Prefix to Remove - prefix to remove from number if number matches the rule. You can incorporate a rule into the remove prefix string. E.g. if number +16778585657 matches rule specified in Match Number and Remove Prefix rule is +1677 then result number is 8585657.
Intl Prefix Substitution - replace + with some string, such as 00.
Prefix to Add - prefix to add to number .
Reg Timeout - defines the refresh interval for your SIP registration. Value in seconds. Minimum 60, default 180.
Incoming Calls - when enabled, this account will be registered with your VoIP provider allowing incoming calls; otherwise this account can only be used to make outgoing calls.
Protocol (UDP / TCP / TLS) - Most VoIP providers only support UDP transport. TCP and TLS transports will allow you to take advantage of the iOS4 multi tasking support i.e. to be able to receive incoming calls while the app is sleeping in the background. This preserves battery. For UDP you need to enable the Run In Background setting and disable "Save battery mode" to support incoming calls when in the background. To encrypt signaling messages, choose TLS (if supported by your VoIP service provider). Default "UDP".
Encrypt Audio (Never / If Possible / Always) - this setting is used to encrypt audio using SRTP. "If Possible": the call will be encrypted only if the remote party accepts encryption (otherwise it will be unencrypted). "Always": the call will be encrypted and will succeed only if the other party accepts encryption; otherwise the call will fail. If call is encrypted, "call media encrypted" appears on the call view. Default "Never".