NAT over PPTP issue with SIP provider

Hi,
I have the following problem and I request your help:
I use pptp to connect to the internet. I use source-nat masquerade and I have full connectivity except an issue with SIP. I cannot register to my sip provider.

When I am connecting to the internet using ADSL and PPPoE, with the same nat rule I have no issues at all.

the nat rules I use are:
/ip firewall nat
add action=masquerade chain=srcnat comment=“” disabled=no out-interface=
pptp-prov

thanks for your input.

do you have SIP helper enabled? does this SIP provider use udp/5060 for control channel?