ORANGE (20801) & Bouygues (20820)
 

do ORANGE (20801) & Bouygues (20820) allow voip calls when using data through their mobile networks?

I can only speak for Bouygues B&You.
1. Skype worked fine.
2. SIP and PPTP were blocked.
3. OpenVPN over TCP/443 worked great.

skype uses other data transmission protocol than sip?

Yep

Skype: Skype protocol - Wikipedia, the free encyclopedia

SIP: Session Initiation Protocol - Wikipedia, the free encyclopedia

hello,

B&you is a MVNO of BoT (20801) and i confirm you can do skype officially
for BoT the #3 number operator offer it depends of the offer.

for orange same thing as Bot, they have MVNO or even full-MVNO on this core cell network.

next time please only use real name offer and forget to speak about 208xx number.

FYI = BoT = bouygues telecom well-known acronym/shortcut

so any french network that allows SIP?

it depends of the MNO/MVNO/full-MVNO offer.