iPhone only rings one when call comes in.
When I get a call on my iPhone it only rings once then goes silent
First try restarting the phone.
Is it going to voicemail greeting?
If i it rings once then goes to voicemail, then it is timing out.
Google how your particular provider increases the ring time,
or try dialling this code in phone app
**61*vvv*11*XX#
where vvv is voicemail number
XX is the seconds of ring time you want, up to 29 max