He has faith in his female comrade also luffy and zoro does but without risking their lives by putting her in necessarily damage. Just because robin is a badass and reliable doesnt mean he should put her in harms way if it was possible to avoid it.
They are pirates.
They are against a Yonko's alliance.
They are targeted by the WG and the Navy.
They are going for the One Piece and will eventually clash against other powerful enemies.
They are ALL in harm's way already. :P
Let's assume that Sanji doesn't call Robin.
What happens?
The Calamities basically take over. Marco, who held King and Queen, is currently going against King only. Queen is missing. Who knows what this fucker is planning. Jack is quite well and is going for the Scabbards. The alliance doesn't know where they are, aside from Sanji.
If he doesn't call for help, the scabbards die, the "toki or hyori" there will die as well. Yamato will eventually be found and overwhelmed by the Calamities and Momo will be killed. He HAS to get out of there. Doesn't matter if you like him or not. He's the third strongest SH. But the 2 strongest are on the roof busy against 2 yonkos, so Sanji is THE STRONGEST SH NOT ON THE ROOF. He's a key asset for the alliance to be able to push back the All Stars.
Now, we can argue all we want on how many ways he can get out of there. But Oda decided to go for the route "he only gets out of there, IF he calls for Robin". Oda didn't make him use Raid Suit, or Skywalk, or his flames or whatever. It's either "call for help or die".
I think that intentionally ignoring all the ways the character should be able to accomplish something, just for the sake of "plot" is bad, but it is what it is.