(1a) He is back on the phone talking to his mother.
If it works,how do you parse this sentence？
This question of yours, Robby zhu, has occasioned a fair amount of reflection on my part over the last couple of days. I don't believe I'd ever thought about (or read about) how to parse the relationship of the participial phrase to the rest of the sentence in this type of construction, which is extremely common among native speakers, and thus very much worth parsing.
Omitting "back," here are some other examples, with other prepositions:
(2a) He is in the bathroom taking a shower.
(3a) He is under the bridge eating a sandwich.
(4a) He is across the street buying a newspaper.
(5a) He is around the corner smoking a cigarette.
While I am sympathetic to the analysis Gustavo has proposed, which gives primacy to the (undeniably focused) prepositional phrase and interprets the participial phrase as supplementary, adverbial information, I have come up with an alternate analysis that provides a straightforward way of parsing the sentence.
Somewhat tentatively, I parse such sentences as containing progressive verb phrases with prepositional-phrase modifiers that are fronted within the (lower) verb phrase, appearing on the left side rather than on the right side. From a formal syntactic standpoint, progressive verb phrases consist of two verb phrases, one headed by the auxiliary verb "BE" and the other by the main verb.
On my parsing, then, the prepositional-phrase adjunct modifier is an adjunct in the lower verb phrase (headed by the main verb). Normally, that prepositional phrase appears on the right side; however, in this special type of sentence, the prepositional phrase is given focus and is thus fronted within the verb phrase. It flip-flops to the other side.
Below are the sentences from which I take the examples under discussion here to be derived. I have also attached a picture of two syntax trees that I have drawn to illustrate the derivation. Please do not worry about the labels of the various nodes! What I would like to draw your attention to is the red node and its two daughter nodes, also colored red. The two daughter nodes simply flip-flop in the second tree and that converts the straightforward progressive construction to the less straightforward construction that Robby zhu has asked about.
(1b) He is back talking to his mother on the phone.
(2b) He is in the bathroom taking a shower.
(3b) He is eating a sandwich under the bridge.
(4b) He is buying a newspaper across the street.
(5b) He is smoking a cigarette around the corner.