Different SIP gateway have different behavior. And different VPN gateway have different behavior too.
We have tested following VPN tunnel:
Netscreen NS25 <--- IPSEC VPN ---> Draytek 2950
Netscreen 5XP <--- IPSEC VPN ---> Draytek 2950
Draytek 2200E+ <----- PPTP VPN ----> Draytek 2950
Linksys VPN <--- IPSEC VPN ---> Draytek 2950
ZyXel VPN <--- IPSEC VPN ---> Draytek 2950
All should be fine for remote SIP gateway. We only found Weltech WG3804A one way audio under Netscreen 5XP <--- IPSEC VPN ---> Draytek 2950. However, replaced GXW4104 or MP-114 are works fine. It should be the WG3804A's issue. For Patton also works fine, we already tested SN4960 T1/E1/PRI/30V. SN41XX FXO will be test later.
I prefer Patton's voice quality. However, the config should be not easy.