No there are just few conventions:

1) the extension must be numeric es. [123]
2) the callerid must be in the format "name <extension>"

example in you SIP or IAX.conf:

[123]
host=dynamic
qualify=yes
notransfer=no
secret=xxxxxxxxxx
type=friend
auth=md5
context=your extensions context
dtmfmode=rfc2833
username=123
callerid=pippo <123>
mailbox=123@your voicemail context
disallow=all
allow=gsm
deny=0.0.0.0/0.0.0.0
permit=192.168.1.0/255.255.255.0

If you use Voice Mail a VM context must exist (see FAQ I don't have VM notification on the Phone)

In the IAX and SIP conf the user that use voicemail must have the definition parameter "mailbox" for example 'mailbox=123@yourVMcontext' (see FAQ I don't have VM notification on the Phone)