no ring tone

I sent Linksys ATA to my parents in Europe and very often when i call them there is no ring tone at all, and after 30 second there is a message "the person you are trying to reach is not available, please leave a message". When i do get a ringtone and they answer we have normal conversation without a problem. But very often i get no ring tone at all and if i keep trying I may get lucky and get ring tone suddenly and have normal conversation. Could it be their router problem (i do not know what router they have) may be porting problem? When i look at their FPL call logs all those calls that i tried without ring tones are registered as "normal call clearing".
Here are their ATA details:
SIP Status: connected
SIP User Agent: Linksys/PAP2-3.1.22(LS)
Inbound/Outbound Proxy: voip.freephoneline.ca
Registration Expires: 3600
Use Preferred Codec: G711U or G729a
Use Preferred Codec Only: No
RTP Packet Size: 0.020
Ring Tone Frequency: 52
Ring Tone Voltage: 90
Recommended Linksys Dial String: (911|[2-9]xxxxxxxxx|1xxxxxxxxxx|011xxxxxxxxxxxx.|98*|[6-7]x*xxxxxxxxxxx.)
(please note that this is a Linksys dial string we use for our devices and may not work with third party, non-Linksys/Sipura devices)
Any suggestions?
Here are their ATA details:
SIP Status: connected
SIP User Agent: Linksys/PAP2-3.1.22(LS)
Inbound/Outbound Proxy: voip.freephoneline.ca
Registration Expires: 3600
Use Preferred Codec: G711U or G729a
Use Preferred Codec Only: No
RTP Packet Size: 0.020
Ring Tone Frequency: 52
Ring Tone Voltage: 90
Recommended Linksys Dial String: (911|[2-9]xxxxxxxxx|1xxxxxxxxxx|011xxxxxxxxxxxx.|98*|[6-7]x*xxxxxxxxxxx.)
(please note that this is a Linksys dial string we use for our devices and may not work with third party, non-Linksys/Sipura devices)
Any suggestions?